bug#40317: 27.0.90; Reverting a buffer that visits C file signals an error

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Sep 19, 2020 at 10:35:18 +0300, Eli Zaretskii wrote:
> > From: Jeff Norden <jnorden@tntech.edu>
> > Cc: 40317@debbugs.gnu.org, eliz@gnu.org,
> >   damien@cassou.me
> > Date: Fri, 18 Sep 2020 17:03:07 -0500

> > Somehow, and I sure don't know how, I think that c-after-change gets
> > called with: c-new-END already set to the value of point-max after the
> > insertion; and with the other variables set so that that beg, end, and
> > old-len remain unchanged.  It's the only scenario that I can see that
> > fits the backtrace that Eli posted.

> > If Damien and/or Eli can temporarily try out the test that I suggested
> > and get it to trigger, I think that would verify this.  In fact, maybe
> > warn would be even better: 

> >   (if (> c-new-END (point-max))
> >     (warn "c-new-END is too big! %d > %d" c-new-END (point-max)))

> Unfortunately, the problem no longer happens to me, not in many
> moons.  Not sure why: I didn't change my usage patterns.

The reason is the following patch, which was committed slightly before
you reported the bug, but before you had updated your Emacs to include
it:


commit a3c2d186eb514b505e61c2a89a1df886dbfcb06b
Author: Alan Mackenzie <acm@muc.de>
Date:   Wed Mar 4 21:17:04 2020 +0000

    CC Mode: Fix the handling of two adjacent after-change-functionses.

    The bug involved failing to set c-new-END correctly, which lead to an
    args-out-of-range error when after-change-functions was invoked twice without
    an intervening invocation of before-change-functions.

    * lisp/progmodes/cc-mode.el (c-after-change): Correct a coding error in the
    handling of c-just-done-before-change.


What triggered the bug there was insert-file-contents not calling
before-change-functions when called from revert-buffer.

> Hopefully, Damien will be able to test this theory.  Thanks.

What Damien has found appears to be a bug distinct from the one you
reported in March.

-- 
Alan Mackenzie (Nuremberg, Germany).